Everybody loves shaders, and Unity’s 3.X ShaderLab http://unity3d.com/support/documentation/Components/SL-Reference.html is interesting in what they have done (ShaderLab is similar to CgFX scripts, but unique to Unity).
Unity 3.X has updated their rendering model, and specifically changed how to write shaders for Unity 3.X. The newly structured surface shaders, which through a combination of ShaderLab syntax (often pragma settings) and Cg progamming language allowed shaders to be created that would work with both Unity’s forward and deferred rendering lighting model. The pragma basics include the Lambert and BlinnPhong lighting models as well as more complex bump, cubemap and emission shader properties and logic. The Unite talk I went to also mentioned a specific “gotcha” related to Unity’s use of the specular color, requiring the specular color parameter to be specifically named _SpecColor to allow Unity access to the separate specular highlight (this appears to be a hardcoded issue, but is different from the Unity 2.X standard).
As a whole Unity is still rendering average scenes with minimal lights much faster in forward rendering (at least in the test scenes that I have been working with). The deferred rendering will likely be useful for very specialized scenes with lots of lights, but it is nice to have options.
Unfortunately the biggest issue for large scale projects is the incompatibility between Unity 2.X shaders and Unity 3.X. If you have written custom shaders, and have exported assetBundles using those shaders, you will have to rewrite the shader using Unity 3.X techniques and re-export all the assetBundles (hopefully you have an automated build process, for your assets to help with this, but not everyone will). If you notice assetBundles brought in with bright pink colors, it means that your shaders are incompatible and will need to be updated (which makes QA much easier on these assets).
Overall, there is not turning back, and you should strive ahead using Unity 3.X and all the other great features it has, the ShaderLab language is much simpler and will allow more artists to contribute their ideas for shaders and will greatly improve writing time and debugging of shaders in general. Still, would have been nice if the shaders were backwards compatible and not pink 😛
Hopefully you have minimal pink,